Bubble sort is a simple comparison-based sorting algorithm that repeatedly steps through the array, compares adjacent elements, and swaps them if they are in the wrong order. In the worst case, where the array is sorted in reverse order, each pass of the algorithm will result in the largest unsorted element "bubbling" to the end of the array. This requires n-1 comparisons in the first pass, n-2 in the second pass, and so on, resulting in a quadratic time complexity of O(n^2).